Reionization, Environment, and the Galaxy Content of Milky-Way Mass Halos

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Dr. Wechsler and her team will explore how hydrogen gas is re-ionized by the ultraviolet radiation from the earliest stars and active galactic nuclei, and how that affects star formation in small galaxies. The standard Cold Dark Matter cosmologies predict many more low-mass dark halos, clumped around much larger dark halos, than we observe as satellites of larger galaxies. But ultraviolet radiation from the larger galaxy may prevent gas in the smallest satellite dark halos from collapsing to form stars, so that no galaxy is born there. Dr. Wechsler's work will improve on current studies, which assume that the entire cosmos reionizes at a single time. She and her team would first run large cosmological N-body simulations including only the dark matter. They will then examine how changing the level of ultraviolet radiation alters the number of infalling subhalos that could host faint satellite galaxies. The team will then use the large-scale simulations, together with models for reionization, to estimate the range of possible ultraviolet radiation levels around a galaxy like our Milky Way. The calculations will predict how patchy reionization affects the expected satellite population of Milky-Way-sized galaxies, and examine what relations exist between a galaxy's reionization history and its satellite population, formation process, and environment.A Hispanic postdoc and a graduate student will be trained as they participate in the research. Students from under-represented groups in a program at Dr. Wechsler's institution will also be involved. She will continue to participate in public outreach lectures and in developing curriculum material for high schools.